FIG. 3 shows a partially sectional perspective view of a conventional electrolytic capacitor. Capacitor element 29 is formed by positive foil 21 and negative foil 22, wound together with intervening separator 23. Positive foil 21 is an aluminum foil, the effective surface area of foil 21 has been enlarged through etching process, and a dielectric oxide film formed through a chemical process is provided on the surface. Negative foil 22 is also made of an etched aluminum foil. Positive foil 21 and negative foil 22 are connected, respectively, with positive lead 25 and negative lead 26. Capacitor element 29 is impregnated with electrolyte 24 for driving. Capacitor element 29 is inserted in metal case 28 of aluminum, etc., and case 28 is sealed with sealing plate 27 made of rubber or the like material to complete a finished electrolytic capacitor.
An ionic conductive liquid is used for electrolyte 24; for example, an organic solvent of ethylene glycol, γ-butyrolactone, etc., in which boric acid, ammonium borate, etc. is resolved as solute. The content of water in electrolyte 24 is reduced, when solute of azelaic acid, 1,6-decane dicarboxylic acid, 5,6-decane dicarboxylic acid, dioic acids having a side chain or nonaqueous salts of the acids are used. Using such solute, the unclosing of electrolytic capacitors due to an increased inner pressure caused by the water content in the environment hotter than 100° C. can be curtailed.
There are other electrolytic capacitors which use an electronic conductive solid electrolyte in place of liquid electrolyte 24. Polypyrrole, polythiophene, polyaniline, etc. are used as the electrolyte. These electrolytes offer the advantages of lower equivalent serial resistance (hereinafter referred to as ESR). Thus it implements an electrolytic capacitor having a low ESR and a superior impedance characteristic.
There is still other proposal of using an ionic conductive solid electrolyte; the electrolyte is divided into the inorganic and the polymeric. The inorganic electrolyte has the advantage of high ionic conductivity; however, it is heavy, inflexible and less plastic. On the other hand, the polymeric electrolyte is attracting the attentions because it is light in weight and has superior mechanical properties such as flexibility, plasticity, etc.; despite its disadvantage in the lower ionic conductivity as compared to the inorganic electrolyte.
In the electrolytic capacitors using the ionic conductive polymer electrolyte, gelled electrolyte layer is formed between a positive foil having dielectric oxide film and a negative foil. Japanese Patent Unexamined Publication No. H09-082580 discloses an electrolytic capacitor using the gelled electrolyte of the following ingredients:                (a) a thermoplastic elastomer selected from among the polyamide-polyether block polymer and polyester-polyether block polymer,        (b) a polar organic solvent, and        (c) a solute.According to Japanese Patent Unexamined Publication No. H10-223481, a positive foil and a negative foil provided at the surface with pits having the diameters larger than 0.1 μm are wound together with an intervening separator containing polyvinyl alcohol, for forming a capacitor element. The capacitor element is put into contact with a liquid electrolyte containing ethylene glycol for electrolytic capacitor, and the liquid electrolyte is gelled.        
Electrolytic capacitors for use in nowadays countermeasure circuits against higher harmonics and in vehicles are requested to have a higher withstanding voltage, a higher heat resisting property, a longer life and a higher reliability in the anti-vibration capability, as compared to conventional electrolytic capacitors. The conventional liquid electrolytes using ionic conductive liquid can not satisfy the requirements in terms of the higher withstanding voltage ratio (improved spark voltage), the higher heat resisting temperature and the longer life.
The ionic conductive polymeric electrolyte has a high withstanding voltage, but the ionic conductivity is low as compared with the liquid electrolyte. As the result, resistance of electrolyte itself is high, which leads to a greater resistance loss as a capacitor. Therefore, it is important to reduce the resistance of the separator. Manila papers, kraft papers, esparto papers, etc., which are the materials generally used for the separator in aluminum electrolytic capacitors, have a high density. If, a lower density version of these materials is used for the purpose of lowering the resistance, the anti-short circuiting capability and the tensile strength are not enough.